Some thermal processing furnaces for workpieces such as steel materials, having thermal conductivity by heating, soaking, or cooling the workpieces include a blowing hood, and are configured to blow hot air or cold air as a gas flow from a nozzle provided in the blowing hood.
For example, a “continuous heating furnace” in Japanese Patent Laid-Open No. 2009-57621 is a heating furnace which heats and soaks a steel material by continuously conveying the steel material, the continuous heating furnace including a combustion burner, a fan that circulates a flue gas within the furnace, a partition plate that covers a steel material conveyance path and guides the flue gas from a furnace bottom to its top, and a slit plate that regulates the flow of the flue gas above the steel material conveyance path and below the partition plate, wherein a slit width of the slit plate changes in a steel material conveying direction. Thus, the continuous heating furnace has excellent temperature rising and furnace temperature distribution characteristics. The slit corresponds to the nozzle, and the steel material is conveyed by a walking beam.
In the past, a steel material conveying surface of the walking beam and the slit plate where the slit blowing a gas flow is formed have a constant distance relationship. Therefore, roughly speaking, a distance between the steel material and the slit varies depending on the magnitude of the dimensions of the steel material on the steel material conveying surface. To be more specific, there is such a distance relationship that a steel material having a large height dimension is located close to the slit, and a steel material having a small height dimension is located far from the slit.
The gas flow of the flue gas blown from the slit has a high flow velocity immediately after being blown out, while the gas flow is diffused and the flow velocity is decreased with distance from the slit. When the steel material is thermally processed by causing the gas flow to impinge on the steel material, heat is more efficiently transferred from the gas flow to the steel material as the flow velocity is higher, that is, as the distance between the steel material and the slit is smaller.
Based on the above description, a steel material having a smaller height dimension is located farther from the slit so that the flow velocity of the gas flow impinging on the steel material is decreased, and it is difficult to ensure sufficient heat transfer. Therefore, it takes a long time until the steel material is heated to a desired temperature.
When a thermal processing furnace that handles steel materials of various dimensions is designed, it is necessary to determine a height dimension from a steel material conveying surface to a slit plate located above the surface based on the dimensions of a tallest steel material. Meanwhile, it is also necessary to determine a heating time required to heat the steel material to a desired temperature based on the dimensions of a shortest steel material with poor heat transfer. Thus, to ensure the heating time, a facility having large heating capacity is required, and a large facility space is required since the furnace is extended in a conveying direction.
It could therefore be helpful to provide a thermal processing furnace for workpieces having a blowing hood in which a nozzle is installed, the nozzle blowing a gas flow to perform thermal processing such as heating, soaking, and cooling on the workpieces, and to provide a thermal processing furnace for workpieces capable of efficiently performing thermal processing by causing a gas flow having a high flow velocity to impinge on the workpieces regardless of the dimensions of the workpieces, thereby contributing to space saving and energy conservation.